414 PROCEEDINGS OF THE ACADEMY OF [May, 



is the distinct coronary vein, lying in a slight depression between 

 the right and left ventricles. 



From the anterior regions of the body the blood is brought back 

 through two fairly wide but very thin-walled precaval veins which 

 pass across the dorsal surface of the heart to enter the sinus venosus. 



The arterial blood is brought from the lungs by two wide, thin- 

 walled pulmonary veins, Plate XIII, fig. 4, v.p.d., v.p.s. They 

 leave the lungs somewhat caudad to their middle region, near the 

 point of entrance of the bronchii and the pulmonary arteries, pass 

 mediad in a direction almost at right angles to the long axis of the 

 body, and enter the left auricle at the same point. 



Blood leaves the heart through five large vessels: (1) the pul- 

 monary artery, (2) the two aortic arches, (3) the right subclavian, 

 (4) the primary carotid. 



The pulmonary leaves the small right ventricle as a single stem, 

 which soon branches into two arteries that pass cephalad and laterad 

 to the lungs, along with and close to the main bronchi. The other 

 arteries that carry blood into the systemic circulation are fused at 

 their base to form a sort of conus arteriosus which may be distended 

 in injected specimen until it is larger than the two ventricles together, 

 When opened this conus is found to contain two chambers that lead 

 into the left ventricle; the larger chamber gives origin to the right 

 systemic arch, the right subclavian, and the primary carotid; the 

 smaller chamber is the basal part of the left systemic arch. 



The two systemic vessels, fig. 4, Ao.s, Ao.d, pass, in the usual 

 manner, as two arches to the dorsal region, just posterior to the 

 ventricles, where they form the dorsal aorta in the manner to be 

 described in connection with the arterial system. 



The further course of the primary carotid and of the right sub- 

 clavian will also be described in connection with the arterial system. 



The auricles are very large in proportion to the ventricles, though 

 their relative sizes will, of course, vary with the amount of contained 

 blood. 



The Venous System. 



The Posterior Vena Cava and its Branches. 



The postcava, fig. 1, pc, as noted above, is a wide, thin-walled 

 vessel seen extending across the short space between the anterior 

 face of the right lobe of the liver and the sinus venosus. As was 

 also noted above, the hepatic veins, vh — at any rate that from the 

 left lobe of the liver — enter the postcava so close to the heart that 



